The present invention generally relates to an improved method and apparatus for coherent optical information processing, and in particular to the reduction and elimination of the measured intensity variations in spatial patterns due to laser speckle and/or propagating-media-induced spatial or temporal phase shifts. The invention has application for coherent optical imaging, free space optical communications (FSO), and anywhere an incoherent, narrow bandwidth, optical source is needed
Speckle and scintillation have been limiting factors in information transfer with coherent light sources since the advent of the laser. Speckle is the mottled light intensity pattern that results when a laser is reflecting off a non-specular surface. In an imaging system the mottled intensity pattern overlays the actual image and degrades quality.
Scintillation causes fluctuations in laser beam power resulting from transverse phase variations in the wavefront. This effect is most commonly encountered in free space laser propagation. The atmosphere can be seen as being made up of many small pockets of turbulent air, each having slightly different refractive index properties. As a laser beam propagates, these pockets act as weak lenses which deflect the light slightly and cause random transverse path length differences. This gives rise to coherent combinations which are seen by a detector system as power fluctuations. A variation in power at the detector can result in a loss of information.
These phenomena are a result of the coherent nature of laser light, and are inherent to any system employing a coherent source. For free-space laser communications scintillation effects are the major driver for determining the transmitter power in a single transmitter communications system operating in clear air. The excess power that must be transmitted to keep the burst error rate above 10−6 is between 300 and 1 million times the minimum power that would be required to maintain this error rate in the absence of scintillation (Kim et al., “Scintillation Reduction Using Multiple Transmitters,” SPIE vol. 2990, pp. 102-113, 1997). Many patents and scientific papers have addressed the problem of speckle and scintillation by affecting the spatial or temporal coherence of the source. U.S. Pat. Nos. 4,961,195 and 5,048,029, by Shupsky et al., require a broad bandwidth laser source. The plurality of frequencies contained in the source is exploited to give rise to coherent combinations, which change so fast over the spatial extent on the beam, that the detection system time averages the signal making it seem incoherent. Any power fluctuations due to scintillation are mitigated. The drawback to this technology is the fact that a broad bandwidth source is required, an unattractive feature for FSO.
A similar method and arrangement is prescribed in U.S. Pat. No. 6,738,105 B1, by Hannah et al. In this arrangement speckle reduction is achieved by propagating a coherent light source through a mechanically rotated random phase plate. The scope of the invention described is limited to detector integration times comparable to that of the human eye, nominally 60 Hz. A random phase plate creates a large number of path length differences in the transverse extent of the beam such that the intensity fluctuations occur on a very small scale. Rotation of the phase plate shifts the position of these fluctuations so as to be averaged by the detection system. The arrangement does not address speckle issues and does not apply to FSO and imaging systems which require large bandwidth information capture.
Spatial phase modulation is also demonstrated in U.S. Pat. No. 6,898,216 B1, by Kleinschmidt and U.S. Pat. No. 6,952,435, B2, by Lai et al. The methods and apparatuses described in these arrangements require a mechanically manipulated element which reduces spatial coherence. The mechanical nature of the solutions rendered these methods unsuitable for many FSO implementations.
U.S. Pat. No. 6,863,216 B2, by Tsikos et al., also addresses this issue with spatial phase modulation, but is very general in how that modulation is realized. The method described reduces speckle for a planar laser illumination and imaging based camera system, used in illuminating moving and stationary objects. However, Tsikos fails to discuss temporal phase modulation by use of myriad modulating techniques, and applies the technology to a specific laser system that is not applicable to FSO.
Currently the most popular method of speckle reduction for FSO is aperture averaging. The general concept is to increase the size of the receiving aperture so that no power is lost due to intensity fluctuations caused by scintillation effects. The most common methods for dealing with atmospheric turbulence of which aperture averaging is one is discussed by Anguita (J. A. Anguita, et al., “Multi-Beam Space-Time Coded Systems for Optical Atmospheric Channels,” Proc. of SPIE, Vol. 6304 63041 B, 2006). The drawbacks are as follows. Scaling aperture size is costly and often times inconvenient. Furthermore, a larger aperture requires a larger detector. As of late aperture averaging has been implemented with multiple signal transmission elements. This arrangement exploits the effect of scintillation causing a higher density speckle pattern at the receiver, in essence a reduction in spatial coherence.
The increase in computing power has allowed digital image processing techniques to be developed for speckle reduction. This art generally relies on a wavefront detection scheme and then an optimization metric to manipulate the source and compensate for the atmospheric turbulence. In the scientific paper by Khandekar et al. a feedback system is described when a wavefront is measured, analyzed and compensated for with a liquid crystal spatial light modulator (SLM) (R. M. Khandekar, et al., “Mitigation of Dynamic Wavefront Distortions using a Modified Simplex Optimization Approach,” Proc. of SPIE Vol. 6304, 63041J, 2006). The process solves the problem of scintillation by applying the inverse of the scintillation obtained in signal propagation at the transmitted source. The drawback to this specific technology is related to the speed at which the liquid crystal SLM can be adjusted. Also the method is slowed by the computer algorithm used to dissect each the aberrant wavefronts.
The prior art has also reduced scintillation and speckle effects by changing the temporal coherence of the source. The general idea is to make the coherence time of the laser less than the integration time of the detector. Implicit in this solution is the generation of a broadband source. The drawbacks of having a wide bandwidth source in FSO are obvious and current research is almost entirely directed toward spatial averaging and digital information processing. This technique and those described above, as well as many others are discussed in the scientific paper by Iwai and Asukura, published in the Proceedings of IEEE, Vol. 84, No. 5, May 1996.
The prior art generally fails to solve the problem of speckle and scintillation for the diversity of applications to which these issues apply. Thus there is a need for a method that is applicable to any coherent optical information process where speckle and scintillation are limiting factors, while avoiding the shortcomings of the prior art.